From Lake Elsinore, ancient tales of climate change

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As the glaciers of the last ice age retreated across North America, the region that would become Southern California experienced two sharp drops in available freshwater – the result of climate change driven in part by rising greenhouse gases, according to a newly published study.

And while the study, led by a Cal State Fullerton researcher, does not attempt to project future climate effects, it does reveal an ancient record of sudden, climate-driven changes in water supplies.

The slower, natural buildup of greenhouse gases in the atmosphere, as the last ice age closed, also could have implications for how similar changes might unfold amid the much more rapid, human-driven buildup taking place today.

The new data emerged from cores of sediment drilled from the bottom of Lake Elsinore, offering glimpses of the Southern California environment tens of thousands of years ago.

“Different lakes tell us different stories,” said Matthew Kirby, an associate geological sciences professor at Cal State Fullerton and lead author of the study. “Some tell us really good stories about floods, some tell us really good stories about droughts, some tell us really good stories about changes in vegetation.

“By looking at these sites, we've put together this remarkable picture of what the Southern California climate was like for the past 33,000 years.”

In this case, Kirby and his team focused on Lake Elsinore's tales of rainfall and river flows: where the rain was coming from, and how strongly the San Jacinto River was flowing.

At two critical points, 14,700 and 12,900 years ago, freshwater flows dropped suddenly. And in each case, these dramatic shifts took less than 70 years.

“That is actually something, in our lifetime, that we would notice,” Kirby said. “We're talking about fundamental shifts in the state of the climate. Once it shifted, it didn't return back to pre-existing conditions.”

Kirby and his fellow researchers used two proxy measurements to detect past climate shifts. One was the number of sand grains in the sediment cores; more sand equals stronger water flows.

The other involved the tracking of hydrogen isotopes – different forms of hydrogen atoms – found in the wax of leaves that washed into the lake and became trapped in the mud at its bottom, eventually becoming part of the deep sediment.

Lighter versions of hydrogen atoms are found in rainstorms originating in the North Pacific, while the heavier version, known as deuterium, is more common in monsoon moisture from the south.

“Different storms from different regions have characteristic fingerprints,” said Sarah Feakins, an assistant professor of earth sciences at USC and a co-author of the study.

The two methods also complement each other. The sand-grain measurements reveal abrupt shifts, while the leaf-wax method reveals more gradual changes.

As the glaciers retreated and greenhouse gases built up, storm tracks from the subtropical Pacific came to dominate Southern California – an effect that continues today, Kirby said.

The region also became significantly drier between 19,000 and 9,000 years ago.

Separate studies using computer models of climate have shown that Southern California appears to be in for another drying episode as the climate warms.

“I think the caution is, we are going to get drier,” Kirby said. “Our practices of water in Southern California will have to be more like the water practices in Phoenix – less natural grass, less water-dependent vegetation.”

And if the past is any indication, sharp drops in freshwater availability can happen suddenly, in a matter of decades – perhaps causing further strain in a region that is already experiencing tightened supplies.

Those same climate models also might need updating, Kirby said. Modelers trying to predict future climate effects typically consider Southern California and the rest of the Southwest as a single unit, governed by the same climate forces, he said.

But while both areas are strongly influenced by large-scale changes in North Atlantic ocean circulation, winter storms and monsoons from the south prevail in the interior Southwest, such as Arizona and New Mexico, while winter storms alone dominate in the coastal Southwest.

“We need to start to break apart these regional patterns of climate change,” Kirby said. “We need to talk about climate change on very specific scales.

“What makes Southern California really specialized is it's a winter-dominated hydrological regime,” he said. “You can't just say what happens in the interior Southwest is the same thing that happens here.”

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